Characterization and reactivity of Mn-Ce-O composites for catalytic ozonation of antipyrine

Article abstract of DOI:10.1039/c5ra11360a

Mn-Ce-O composites were prepared by a redox-precipitation method and used as a catalyst for ozonation of antipyrine (AP) in aqueous solution. The phase composition, surface area and electron transfer ability of the products are dependent on the molar ratio of precursors. Mn-Ce-O(8/2) exhibited the highest activity for the degradation and mineralization of AP with ozone, which can be attributed to its high electron transfer ability. The decomposition of ozone into ^.OH can be accelerated through the electron transfer between ozone and catalyst. The refractory intermediates in ozonation and catalytic ozonation were identified and the catalytic performances of Mn-Ce-O(8/2) for these intermediates were also investigated. The hydroxyl radical reaction played an important role in the degradation of the refractory intermediates, and the contribution of surface reactions was strengthened with decreasing pH. A possible mechanism for the catalytic ozonation of AP was proposed.

Full text of DOI:10.1039/c5ra11360a

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DOI: 10.1039/C5RA11360A
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Characterization and reactivity of Mn-Ce-O composites for
catalytic ozonation of antipyrine
Received 00th January 20xx,
Accepted 00th January 20xx
Shengtao Xing, Xiaoyang Lu, Limei Ren, Zichuan Ma*
Mn-Ce-O composites were prepared by a redox-precipitation method and used as catalyst for ozonation of antipyrine (AP)
in aqueous solution. The phase composition, surface area and electron transfer ability of the products are dependent on
the molar ratio of precursors. Mn-Ce-O(8/2) exhibited the highest activity for the degradation and mineralization of AP
with ozone, which can be attributed to its high electron transfer ability. The decomposition of ozone into •OH can be
accelerated through the electron transfer between ozone and catalyst. The refractory intermediates in ozonation and
catalytic ozonation were identified and the catalytic performances of Mn-Ce-O(8/2) for these intermediates were also
investigated. Hydroxyl radical reaction played an important role for the degradation of the refractory intermediates, and
the contribution of surface reactions was strengthened with decreasing pH. A possible mechanism for the catalytic
ozonation of AP was proposed.
DOI: 10.1039/x0xx00000x
www.rsc.org/
particularly, composite metal oxides with mesoporous
structures have been proposed as efficient catalysts for
pollutants removal due to their large active surface and the
synergic effects between different components. However, the
mechanisms of these processes are still unclear, limiting their
application in water treatment.17
Introduction
In recent years, water pollution with pharmaceuticals has
attracted considerable attention because of their potential
impact on human health and the environment even at trace
levels. Pharmaceuticals are released into the environment
through excretion, improper disposal or industrial waste.
Many pharmaceuticals are chemically and/or biologically
persistent, and have been frequently detected in raw water,
drinking water, and treated wastewater.1-4 For instance,
antipyrine (AP) is a widely used anti-inflammatory drug and
has been detected in aquatic environment.5 However, the
removal of AP from water has been seldom investigated. A few
studies reported that traditional water treatment is ineffective
for AP.6,7 Therefore, the development of effective treatment
technologies for AP-induced water pollution is desirable.
Advanced oxidation technologies (AOTs), such as Fenton,
photocatalysis, catalytic wet oxidation and catalytic ozonation,
have been intensively studied for the removal of refractory
organic pollutants due to the generation of strongly oxidizing
•OH radicals in these processes.8-11 Among them,
heterogeneous catalytic ozonation has attracted significant
interest because of its high mineralization efficiency for
refractory organic pollutants and wide working pH range.
Significant efforts have also been made on the preparation of
highly active heterogeneous ozonation catalysts. It has been
found that transition metal oxides with variable electronic
structures can enhance the electron transfer involved in the
catalytic decomposition of ozone into •OH radicals.12-16 In
The efficiency of catalytic ozonation depends on the surface
properties of the catalysts. In aqueous systems, metal oxide
particles can be hydrated and form surface hydroxy groups
which are hypothesized to be active sites in catalytic ozonation.
The surface hydroxy groups tend to dissociate or protonate,
which is dependent on the solution pH. Previous studies
reported that the hydroxy groups both in negative and positive
charge states can accelerate the decomposition of ozone into
•OH.18-20 Moreover, the catalysts usually exhibit high activity
over a wide pH range. This may be due to that molecular
ozone can react as an electrophilic or nucleophilic agent, and
both the highly positively and negatively charged surface may
favour the adsorption of ozone. On the other hand, the surface
charge of catalysts significantly affects the adsorption of
organic pollutants, which is crucial for the surface reactions
involving the adsorbed ozone and pollutants.20-22 Especially,
the adsorption of organic intermediates plays an important
role in the catalytic ozonation process. On the contrary, some
researchers reported that the organic pollutants were mainly
degraded by oxidative process in solution and the adsorption
could be ignored.23,24 The controversies in heterogeneous
catalytic ozonation indicate that the mechanisms of the
process should be further investigated.
Mn-Ce-O composites have been reported to be active for
catalytic combustion of volatile organic compounds, selective
reduction of NOx, catalytic wet oxidation and catalytic
ozonation of wastewaters.21,25-27 Various methods have been
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developed for the preparation of Mn-Ce-O composites, bottom of the reactor. The residual ozone in the off-gas was
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including sol-gel, coprecipitation and redox-precipitation. absorbed by a KI solution. The suspensDioOnI: w10a.1s03m9/aCg5nReA1t1ic3a60llAy
Among them, the redox-precipitation method could improve stirred throughout this experiment. Samples were filtered
the surface texture and active phase dispersion of Mn-Ce-O through a Millipore filter (pore size 0.22 μm) for analysis. The
composites, thus enhancing their catalytic activity.28 Moreover, aqueous ozone remained in the sample was removed by
our previous work found that manganese-based oxides adding an aliquot of 0.1 M Na2S2O3. The same procedure was
synthesized by redox-precipitation method exhibited high adopted for the single ozonation and adsorption without
catalytic activity for organic pollutants removal.20,29,30 ozone. The concentrations of ozone in the gaseous and
Therefore, in the present work, Mn-Ce-O composites were aqueous phase were determined by the iodometric and indigo
prepared by a redox-precipitation method and used for the method (Ozone Standards Committee Method), respectively.
ozonation of AP. The results indicated that the Mn-Ce-O(8/2) The total organic carbon (TOC) of the solution was analyzed
sample exhibited the highest activity for the mineralization of with a Phoenix 8000 TOC analyzer. AP was measured by a high
AP. The structure, surface properties and electron transfer performance liquid chromatography (HPLC, Agilent 1200) using
ability of the catalysts were analysed by different a ZORBAX Eclipse XDB-C18 column (4.6–250 mm, 5 μm) at a
characterization methods, and their correlation with catalytic flow rate of 1 ml min−1, column temperature of 30 °C, and
activity was investigated. The refractory organic intermediates wavelength set of 210 nm. 60% methanol with water mobile
in ozonation and catalytic ozonation were identified. The phase was used. Organic intermediates were determined using
adsorption capacity and catalytic activity of the catalyst a Poroshell 120 SB-Aq column (4.6–100 mm, 5 μm) at a flow
towards these intermediates were evaluated. The possible rate of 0.5 ml min−1. The mobile phase consisted of the
mechanism for the catalytic ozonation of AP was also solution of 0.1% H3PO4/H2O. The results are the averages of
discussed.
duplicate or triplicate experiments.
Experimental
Results and Discussion
Catalyst preparation. All chemicals were of analytical grade and Characterization of catalysts. Fig. 1 shows the XRD patterns of
used without further purification. Deionized water was used in the synthesized Mn-Ce-O composites. For Mn-Ce-O(5/5), the
this study. Mn-Ce-O composites were prepared by a redox- diffraction peaks at 29.0°, 33.8° and 47.9° can be indexed to
precipitation method. A 0.4 mol L−1 Ce(NO3)3 solution was the cubic fluorite structure of CeO2 (JCPDS 43–1002), while
added into a KMnO4 solution under magnetic stirring. The those at 18.1° and 39.3° can be assigned to the tetragonal
resulting solution was maintained at pH 6 and 60 °C for 2 h. phase of α-MnO2 (JCPDS 44-0141). Mn-Ce-O(8/2) and Mn-Ce-
The product was separated from solution by centrifugation, O(9/1) have similar XRD patterns with a broad peak in the 2θ
washed with distilled water for several times, dried at 110 °C range of 25–35° and another much weaker signal in the range
overnight, and further calcined in air at 350 °C for 4 h. The of 40–60°. The weak and broad peaks of Mn-Ce-O composites
products prepared with different precursor ratios (Mn/Ce, x/y) can be due to their poor crystallinity (small crystal size and
were designated as Mn-Ce-O(x/y).
many lattice defects).31
Catalyst characterization. X-ray diffraction (XRD) analysis of
powdered samples was performed with a Bruker D8-Advance
using Cu Kα radiation. The morphologies were examined on a
Hitachi S-4800 field emission scanning electron microscope
(SEM) at 3 kV and a Hitachi H-7500 transmission electron
microscope (TEM) at 80 kV. Nitrogen adsorption–desorption
measurements were performed on a Quantachrome NOVA
4000e surface area and pore size analyzer at 77 K, using the
volumetric method. XPS data were obtained by an AXIS-Ultra
instrument from Kratos, using monochromatic Al Kα radiation
(225 W, 15 mA, 15 kV). The zeta potential of the catalysts in
aqueous solution were measured with a Zetasizer 3000
(Malvern Co., U.K.) with three consistent readings. Cyclic
voltammetry measurements were carried out in a VMP3
electrochemical workstation using
electrochemical cell at room temperature.
a
three-electrode
Catalytic activity measurements. The catalytic ozonatino of AP
was carried out by adding 0.1 g catalyst to 0.5 L of 40 mg L−1 AP
solution in a 1 L cylindrical reactor. Ozone gas (20 mg L−1, 0.2 L
min−1) produced from pure oxygen by a 3S-A3 laboratory
ozonizer (Tonglin Technology, China) was continuously
bubbled into the solution through a porous glass plate at the
Fig. 1 XRD patterns of (a) Mn-Ce-O(5/5), (b) Mn-Ce-O(8/2) and
(c) Mn-Ce-O(9/1).
The XPS spectra of Mn-Ce-O(5/5) and Mn-Ce-O(8/2) were
measured to confirm the chemical states of Mn and Ce. As
shown in Fig. 2, the spectrum of Ce 3d is very similar to that of
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pure CeO2.32 The binding energy of 642.1 eV can be ascribed to aggregated nanoparticles, and their porosityViewcAortuiclledOnlbinee
the 2p3/2 of Mn4+. The result demonstrates that the oxidation attributed to the pores formed between particles.
DOI: 10.1039/C5RA11360A
states of Ce and Mn in Mn-Ce-O(5/5) are +4. For Mn-Ce-O(8/2)
and Mn-Ce-O(9/1), the Mn 2p3/2 peaks shifted to lower values
(641.9 and 641.8 eV, respectively), indicating the formation of
a small amount of Mn3+ species. In this redox-precipitation
method, the phase composition and oxidation states of the
products are dependent on the molar ratio of MnO4− (oxidant)
to Ce3+ (reductant).
Fig. 3 Nitrogen adsorption–desorption isotherms of different
Mn-Ce-O composites.
Fig. 2 XPS spectra of (a) Mn-Ce-O(5/5), (b) Mn-Ce-O(8/2) and (c)
Mn-Ce-O(9/1).
The textural information of the samples was investigated
using N2 adsorption–desorption measurement. As shown in Fig.
3a, all the isotherms are type IV according to the IUPAC
classification, which is characteristic of the mesoporous
structure. The hysteresis loops of Mn-Ce-O(5/5) and Mn-Ce-
Fig. 4 SEM and TEM images of Mn-Ce-O(5/5) (a and d), (b) Mn-
O(8/2) seem to be between Types H3 and H4, while that of
Ce-O(8/2) (b and e) and (c) Mn-Ce-O(9/1) (c and f).
Mn-Ce-O(9/1) is type H4, indicating that the mesopores in Mn-
Surface charge properties of a catalyst are very important
Ce-O(9/1) are more uniform. The surface areas of Mn-Ce-
for the adsorption and catalytic ozonation of organic
O(5/5), Mn-Ce-O(8/2) and Mn-Ce-O(9/1) were calculated to be
about 114, 143 and 156 m2 g−1, respectively. The pore size
pollutants. Fig. 5 shows the changes of the zeta potential with
the solution pH for different catalysts. The point of zero charge
distributions of the catalysts are shown in Fig. 3b. Mn-Ce-
(PZC) of Mn-Ce-O(5/5), Mn-Ce-O(8/2) and Mn-Ce-O(9/1) were
O(5/5) and Mn-Ce-O(8/2) have broad pore size distributions
measured to be about 5.9, 4.9 and 4.7, respectively. The
with average pore diameters of 11.2 and 8.8 nm, respectively,
surface of the catalysts is negatively charged at pH above
while Mn-Ce-O(9/1) displays a narrow pore size distribution
pHPZC, and the negative charge density increases significantly
centered at 6.2 nm. The SEM and TEM images of the
with increasing pH. According to the literatures, pure CeO2 has
composites are shown in Fig. 4. All the samples are irregular
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a much higher PZC than MnO2.33,34 Therefore, the PZC of the
Mn-Ce-O composites decreases with increasing Mn content.
It is generally believed that the decomposition of ozone into
•OH can be accelerated through the electron transfer between
ozone and catalyst.11,35,36 The electron transfer ability of the
Mn-Ce-O composites was examined by cyclic voltammetry. As
shown in Fig. 6, oxidation and reduction peaks were observed
for each sample. The cathodic and anodic peaks can be
attributed to the reduction of MnO2/CeO2 and the oxidation of
Mn2+/3+/Ce3+, respectively.37 The peak potential differences
between the anodic and cathodic peaks of Mn-Ce-O(8/2)
(113mv) and Mn-Ce-O(9/1) (122mv) are smaller than that of
Mn-Ce-O(5/5) (155mv), which can be due to the improved
electron transfer ability. The shape of the curves remained
constant after several cycles, demonstrating the stability of the
catalysts.
View Article Online
DOI: 10.1039/C5RA11360A
Fig. 5 Plot of the zeta potential as a function of pH for different
Fig. 7 (a) Degradation of AP and (b) TOC removal during the
ozonation and catalytic ozonation of AP at pH 6.5.
Mn-Ce-O suspensions in the presence of KNO3 (10−3 mol L−1).
Fig. 6 Cyclic voltammetry scans with different Mn-Ce-O
composites electrodes in a 0.1 M Na2SO4 aqueous solution: (a)
Mn-Ce-O(5/5), (b) Mn-Ce-O(8/2) and (c) Mn-Ce-O(9/1).
Fig. 8 Decomposition of ozone in aqueous dispersions of
various catalysts at pH 6.5.
that the degradation of AP in the catalytic ozonation was due
to the oxidative process in solution. The TOC removal
efficiencies during the degradation of AP are shown in Fig. 7b.
Only 44% of TOC was removed at a reaction time of 60 min in
ozonation. The addition of catalysts significantly enhanced the
mineralization of AP. About 73% of TOC was removed in the
presence of Mn-Ce-O(5/5). Mn-Ce-O(8/2) and Mn-Ce-O(9/1)
Catalytic ozonation of AP. Fig. 7a shows the degradation of AP in
the ozonation and catalytic ozonation processes using various
catalysts at pH 6.5. AP was almost completely removed at 5
min in the absence of catalyst. The addition of Mn-Ce-O(8/2)
and Mn-Ce-O(9/1) accelerated the degradation of AP. Mn-Ce-
O(8/2) exhibited a higher activity: AP was completely removed
at 2 min. AP was hardly adsorbed on the catalysts, indicating
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exhibited much higher activity than Mn-Ce-O(5/5), and their their retention times were 2.5, 2.8, 2.9, 3.4 and 4.3 min,
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DOI: 10.1039/C5RA11360A
TOC removal efficiencies were 87% and 85%, respectively. respectively. The concentrations of these acids changed
Meanwhile, a series of ozone decomposition experiments slightly with prolonging the reaction time, indicating they
were carried out. As shown in Fig. 8, about 86% of ozone was could not be oxidized by single ozonation. As for the catalytic
decomposed within 30 min without catalyst, while the ozone ozonation, formic acid, oxalic acid and succinic acid were
decomposition rate was markedly enhanced in the presence of detected at a reaction time of 30 min. The concentrations of
catalysts. Mn-Ce-O(8/2) exhibited the highest activity and formic acid and succinic acid decreased dramatically after 60
ozone was completely decomposed within 25 min. The ozone min, while the concentration of oxalic acid increased to some
decomposition rates in different catalyst suspensions are extent, which might be due to the decomposition of succinic
positively correlated with the TOC removal efficiencies, acid into oxalic acid.38 The result indicates that oxalic acid is
suggesting that the catalyst can accelerate the decomposition the major refractory intermediate in the catalytic ozonation
of ozone to active radicals. Based on the characterization process.
results, the low activity of Mn-Ce-O(5/5) is related to its small
surface area, high PZC and low electron transfer ability. On the
other hand, Mn-Ce-O(8/2) exhibited the highest catalytic
activity, although its surface area and PZC were between that
of Mn-Ce-O(5/5) and Mn-Ce-O(9/1). The result indicates that
the activity of the catalyst is strongly dependent on its electron
transfer ability.
Fig. 10 HPLC chromatogram of AP aqueous solutions with
different ozonation and catalytic ozonation time: (a) ozonation
30 min, (b) ozonation 60 min, (c) catalytic ozonation 30 min, (d)
catalytic ozonation 60 min.
Previous studies reported that the ozonation rate constants
of formic acid, oxalic acid, acetic acid and succinic acid are only
Fig. 9 TOC removal for six successive catalytic ozonation 5.0, < 4 × 10−2, < 3 × 10−5 and < 3 × 10−2 M−1 s−1, respectively,
experiments with Mn-Ce-O(8/2).
while their •OH rate constant are 1.3 × 108, 1.4 × 106, 1.6 × 107
and 3.1
×
108 M s−1, respectively.31,32 The catalytic
The durability of Mn-Ce-O(8/2) was also studied, and the
result is shown in Fig. 9. The TOC removal efficiencies were all
above 85% in each cycle, and no apparent reduction was
observed after six successive catalytic ozonation experiments.
The above results demonstrate that Mn-Ce-O(8/2) is an
effective and stable catalyst for the degradation and
mineralization of AP with ozone.
Catalytic ozonation of refractory intermediates. Ozonation
exhibited high efficiency for AP degradation but not for TOC
removal, which may be due to the formation of refractory
intermediates. The solution pH decreased from 6.5 to 3.0
during the ozonation process, indicating the accumulation of
organic acids in solution. While the solution pH in the presence
of Mn-Ce-O (8/2) changed less (from 6.5 to 3.5) than that in
ozonation, which might be due to the removal of the produced
organic acids. The major ozonation products of AP in the
ozonation and catalytic ozonation processes were analyzed by
HPLC and identified by comparison with standards. As shown
in Fig. 10, tartaric acid, formic acid, oxalic acid, acetic acid and
succinic acid were detected during the ozonation of AP, and
performances of Mn-Ce-O(8/2) for these carboxylic acids were
investigated. As shown in Fig. 11a, the catalytic ozonation
enhanced the mineralization of these acids compared to
ozonation, and no adsorption of the acids occurred at pH 6.5
expect for succinic acid. The result indicates that the catalyst
accelerated the transformation of ozone into •OH and the
acids were mainly oxidized by •OH in solution. The TOC
removal efficiencies for tartaric acid and oxalic acid
significantly increased with decreasing solution pH (Fig. 11b),
which can be attributed to their enhanced adsorption
efficiencies on Mn-Ce-O(8/2) at lower pH. When the initial pH
decreased from 6.5 to 3.5, the adsorption efficiencies for
tartaric acid and oxalic acid increased to 29.8% and 41.4%,
respectively. The contribution of surface reactions to the
removal of tartaric acid and oxalic acid was strengthened with
decreasing pH. For other acids, the TOC removal efficiencies
changed slightly with decreasing solution pH. The influence of
pH on the catalytic performances of Mn-Ce-O(8/2) towards
different acids may be associated with the catalyst surface
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charge and the dissociation of the acids. The catalyst surface
was negatively charged and most of the acids were dissociated
at pH 6.5. Although the dissociating acids can be more readily
oxidized by •OH,39,40 they were hardly adsorbed on the
negatively charged surface due to the electrostatic repulsion,
thus inhibiting their reactions with •OH generated from the
adsorbed ozone on the catalyst surface. The catalyst surface
became positively charged at pH 3.5, which favoured the
adsorption of the acids containing dissociated functional
groups. Surface reaction involving adsorbed pollutants is
suggested to play an important role in this heterogeneous
catalytic ozonation process.
Conclusions
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Mn-Ce-O composites were prepared by DaOrI:e1d0o.1x0-3p9r/eCc5iRpAi1t1a3t6io0nA
method. The surface properties and electron transfer ability of
the products were dependent on the molar ratio of precursors.
Mn-Ce-O(8/2) was the most effective catalyst for the
degradation and mineralization of AP in aqueous solution by
ozone. The high catalytic activity can be attributed to its large
surface area, low PZC and especially high electron transfer
ability. More organic acids that could not oxidized by ozone
molecule were formed in ozonation compared to catalytic
ozonation. The final intermediate in the catalytic ozonation of
AP was oxalic acid. Mn-Ce-O(8/2) accelerated the
decomposition of ozone into hydroxyl radicals, resulting in
high mineralization efficiencies for the refractory
intermediates. This study offers a new approach for the
optimization of composite oxides with potential applications in
heterogeneous catalytic ozonation.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21207032), the Natural Science
Foundation of Hebei Province of China (no. B2013205028), and
the Foundation of Hebei Education Department (no.
ZD20131005).
Notes and references
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